The present invention relates to the preparation of secondary thiols by reaction of hydrogen sulfide with olefins having carbon numbers in the range from 10 to 30. More particularly, the invention relates to a process in which linear mono-olefins are contacted with H.sub.2 S in the presence of a zeolite catalyst to form a secondary thiol product particularly suitable for use as a precursor in the manufacture of surface active detergent components.
Thiols in the C.sub.10 to C.sub.30 range are known compounds. Secondary thiols prepared by the process of this invention generally have utility in applications for which thiols are recognized in the art (e.g., as odorants, components of lubricant formulations, and curing agents for epoxy resins) but are further of particular advantage when used as intermediates in the synthesis of surfactant chemicals.
It is known in the prior art relevant to this invention that thiols (mercaptans) can be prepared in a process which comprises the addition of hydrogen sulfide to olefins, particularly in the presence of a catalyst, most particularly an acid catalyst. It is further recognized that this process has been applied almost exclusively to the production of tertiary thiols. Markovnikov addition of H.sub.2 S to a "tertiary" olefin, usually an olefin polymer such as a propylene or butylene trimer or tetramer, results in near quantitative selectivity to the tertiary thiol. Exemplification of tertiary thiol preparation from H.sub.2 S and tertiary olefin in the presence of an acidic catalyst is provided by U.S. Pat. Nos. 2,386,769, 2,434,510, 2,464,049, 2,502,596, 2,610,981, 2,950,324, 2,951,875 and 4,102,931.
The preparation of secondary thiols from higher olefins in a similar manner has not been a practical success. In one regard, the preparation of secondary thiols from linear olefins by conventional methods for H.sub.2 S addition has been accompanied by the formation of substantial quantities of dialkyl sulfide by-product. The thiol is the addition product of one H.sub.2 S molecule and one olefin molecule. Dialkyl sulfide is formed when the thiol, once produced, reacts with an additional molecule of the olefin. In known processes for preparing secondary thiols from H.sub.2 S and higher olefins, dialkyl sulfide is formed in a quantity between about 50 and 100 percent by weight, calculated on secondary thiol. (For primarily steric reasons, dialkyl sulfide production is not a significant problem in the preparation of tertiary thiols from branched olefins.) In another regard, the catalysts and reaction conditions which are applied to promote the addition of H.sub.2 S to olefins also promote double bond isomerization and/or rearrangement of the molecular structure of the olefin. As a result, olefins which might be expected to be converted to secondary thiols upon H.sub.2 S addition are instead converted to tertiary thiols.
The recognition in the art of fundamental distinctions in the preparation of secondary thiols on the one hand and tertiary thiols on the other by olefin hydrosulfurization has led to processes in which an H.sub.2 S addition reaction is utilized to separate tertiary olefins, which readily form thiols by addition of H.sub.2 S, from mixtures with other olefins which are significantly less reactive (U.S. Pat. Nos. 2,386,773 and 2,386,769).
The preparation of secondary thiols, particularly secondary thiols of linear carbon chain structure, is most important, if the thiols are to be suitable for use as intermediates in the synthesis of surface active agents. Among the surfactants which can be derived from thiols in the C.sub.10 to C.sub.30 range are the anionic paraffin sulfonates (molecules of the general formula R--SO.sub.3.sup.- M.sup.+, where R is C.sub.10 to C.sub.30 alkyl and M is a cation such as sodium) which are prepared, for example, by oxidizing the thiol. Secondary thiols are particularly useful in the preparation of nonionic thiol alkoxylates (of the general formula R--S(R'--O).sub.x H, where R is again C.sub.10 to C.sub.30 alkyl, R' is C.sub.2, C.sub.3, or C.sub.4 alkyl, and x is an integer between about 1 and 30), which may be prepared by the contact of the thiol with a C.sub.2 to C.sub.4 alkylene oxide at elevated temperature (e.g., 140.degree. C.) and pressure (e.g., 100 psig) in the presence of an acidic (Lewis acid) or a basic (alkaline or alkaline earth metal) catalyst.
One factor of obvious importance to the use of surfactants in detergent service is their capabilities for soil removal. Surfactants produced from secondary thiols have been found to have excellent cleaning properties in a wide variety of detergent applications. Another important factor in surfactant utilization has to do with environmental considerations. In many of their common applications in both industry and the home, surfactants find their way into waste water streams. Biodegradation of the surfactant molecule then becomes of critical concern. Branched carbon chain surfactants derived from tertiary thiols are significantly less biodegradable than those surfactant molecules of linear carbon chain that are derived from secondary thiols, and, accordingly, are much less acceptable for widespread use in detergent and other common surfactant services. For this reason, an improved process for the selective preparation of secondary thiols would be particularly desirable.
With specific regard to catalysts utilized in the process of the present invention, the aforementioned U.S. Pat. No. 4,102,931 describes the use of zeolites to catalyze the addition of H.sub.2 S to branched unsymmetrical olefins for preparation of tertiary thiols. Linear olefins are excluded from the starting material disclosed as useful in this prior art process, and the patent does not attribute to the zeolites any beneficial influence upon any aspect of process selectivity.
Zeolites have also been proposed (U.S. Pat. No. 4,313,006) as catalysts for the conversion of dialkyl sulfide to alkyl mercaptans by reaction with H.sub.2 S at high temperature, e.g., 250.degree.-400.degree. C., preferably 320.degree.-390.degree. C.